There have been widespread reports of very large premium ACA marketplace premium increases in most states in 2018. This analysis provides national estimats for changes in the lowest silver and gold plan premiums between 2017 and 2018. The national average increase was 32.0 perecnt for the lowest-priced silver plans and 19.1 percent for gold plans, but the increases varied by states. Several reasons are behind these large increases. The premium increases reflect significant policy changes and policy debates specifically affecting insurer decisions for the 2018 plan year as well as more typical annual considerations such as trend and healthcare costs. We delineate the factors that contributed to large 2018 marketplace premium increases, provide state-by-state estimates of average premium increases in silver (70 percent actuarial value) and gold (80 percent actuarial value) plans in the marketplaces and provide more detailed analysis of changes in select markets within 20 states.